1. Field of the Invention
The present invention is directed to a pulse sequence for operating a magnetic resonance tomography apparatus as well as to a nuclear magnetic resonance tomography apparatus for implementing the method.
The invention also relates to techniques that employ pulse sequences wherein the magnetization vector in the steady state, given excitation pulses of xc2x1xcex1, oscillates between +xcex1/2 and xe2x88x92xcex1/2. Examples of such pulse sequences are SSFP and FISP pulse sequences, these being explained below.
2. Description of the Prior Art
German PS 44 27 497 discloses a pulse sequence for a magnetic resonance tomography apparatus. This pulse sequence is based on a pulse sequence usually referred to as xe2x80x9cFISPxe2x80x9d (Fast Imaging with Steady Precession). The term xe2x80x9cFISPxe2x80x9d is a known concept in the field of magnetic resonance tomography for a specific pulse sequence and is explained in detail in, for example, E. Krestel, xe2x80x9cImaging Systems for Medical Diagnosticsxe2x80x9d, Siemens AG, Second Edition, 1990, pages 544-547. According to the pulse sequence disclosed by German PS 44 27 497, such a FISP sequence is modified by emitting a radio-frequency pulse in a preparation phase preceding the FISP pulse sequence. This radio-frequency pulse is emitted frequency-selectively and under the influence of a slice selection gradient, so that only one slice of the examination subject is excited. The dephasing caused by the slice selection gradient is in turn canceled by an oppositely directed gradient. The radio-frequency pulse has a flip angle that deflects the magnetization vector, as occurs in the stationary condition of the following pulse sequence. In general, the magnetization vector given excitation pulses of xc2x1xcex1 oscillates between +xcex1/2 and xe2x88x92xcex1/2, and the radio-frequency pulse must then have a flip angle of xcex1/2 with a phase position that is inverted compared to the following radio-frequency excitation pulse.
In the steady state, as stated, the magnetization vector oscillates between +xcex1/2 and xe2x88x92xcex1/2 given excitation pulses of xc2x1xcex1. The spin magnetization represents a problem with regard to achieving rapid imaging, since it is not yet in the steady state at the start of measurement and leads to signal fluctuations between the echoes, i.e. the raw data lines, which produce image artifacts. The method disclosed in German PS 44 27 497 solves this problem before the beginning of the actual FISP sequence by placing the magnetization vector into condition approximating the steady state by a transient response RF excitation pulse.
Another type of sequence known as an SSFP pulse sequence (Steady State Free Precession) is described, for example from E. Krestel, xe2x80x9cImaging Systems for Medical Diagnosticsxe2x80x9d, Siemens Ag, Second Edition, 1990, pages 544-547, that differs from the FISP sequence essentially in that refocusing gradient pulses are employed in all three directions.
The magnetization which persists following the measurement of, for example, an image data set is left out of consideration in this known technique. If a further image data set is acquired at a short time interval from acquisition of an earlier set, the magnetization that arose from the preceding image data set can appear as a noise signal and cause image artifacts.
German OS 198 18 292 discloses a method for controlling a pulse sequence for a magnetic resonance tomography system and an apparatus for the implementation of the method, which achieves flexibly programmable sequence control.
It is an object of the present invention to provide a method in the form of a pulse sequence for operating a magnetic resonance tomography apparatus that makes it possible to acquire a further image in a magnetic resonance tomography scan at a brief time interval following acquisition of data for an earlier image.
This object is inventively achieved in a method for operating a magnetic resonance tomography apparatus wherein image data are obtained according to a pulse sequence wherein, in the steady state, the magnetization vector oscillates between +xcex1/2 and xe2x88x92xcex1/2 given excitation pulses of xc2x1xcex1, and wherein a decay radio-frequency pulse having a flip angle of approximately xcex1/2 with a phase position inverted relative to the last excitation pulse is emitted, whereby xcex1 is the flip angle of the excitation pulses of the pulse sequence.
The decay radio-frequency pulse is emitted at an interval TR/2 following the last excitation pulse, whereby TR is the repetition time of the pulse sequence. The term xe2x80x9crepetition timexe2x80x9d is likewise a known concept in the field of magnetic resonance tomography.
After the end of the pulse sequence and at an interval TR/2 before emitting the decay radio-frequency pulse, a further decay radio-frequency pulse having a flip angle of approximately xcex1 and having a phase position inverted relative to the decay radio-frequency pulse can alternatively be emitted, whereby TR is again the repetition time of the pulse sequence.
In addition to the decay radio-frequency pulse, a gradient pulse having a high product txc2x7Gz can be emitted after the end of the pulse sequence, whereby z is the slice selection direction of the magnetic resonance tomography apparatus and t is the pulse time duration.
According to the present invention, a nuclear magnetic resonance tomography is also provided that has a controller that is programmed for the implementation of the aforementioned steps.